For this case we have that by definition, a variable is a symbol that can be replaced or that takes a numerical value in an equation or expression, mathematical in general.
Then we have the following options:
A)
It is a sum of two real numbers. There are no variables.
B)
In this case the variable is "d"
C)
In this equation, the variable is represented by "r"
D)
It is a sum of two real numbers. There are no variables.
Answer:
A) There are no variables
B) "d"
C) "r"
D) There are no variables